C ++：仅在一个元素上使用unique_copy

Question

This is the implementation as found on cplusplus.com

template <class InputIterator, class OutputIterator>
  OutputIterator unique_copy (InputIterator first, InputIterator last,  
                              OutputIterator result) {
  if (first==last) return result;

  *result = *first;
  while (++first != last) {
    typename iterator_traits<InputIterator>::value_type val = *first;
    if (!(*result == val))   // or: if (!pred(*result,val)) for version (2)
      *(++result)=val;
  }
  return ++result;
}

So when when "first" and "last" iterators point to the same element, we don't return anything? That seems out-of-line with unique_copy's definition: the first element from every consecutive group of equivalent elements in the range [first,last) is copied. Is it because of the "last)" part? Can anyone clarify? Thanks!

Answer 1

You are correct that the [first, last) is the problem.

When assigning iterators to containers, it is standard that the last iterator you can possible have is to the memory position 1 iteration after the last element in the container.

Ex.

vector<int> aVec{5,-8,23,200};
vector<int>::iterator currentItr, lastItr;

// iterator to first element, 5
currentItr = aVec.begin(); 


// iterator to element ***directly after*** last element
lastItr = aVec.end();
// in other words, there is no reason to access the value attached to lastItr

One reason why the .end() standard exists is to easily tell when the currentItr has reached past the usable values in a container;

Ex.

while (currentItr != lastItr) // currentItr will not print once it's equal to lastItr
    cout << *currentItr++ << endl;

In your specific example, if there is one element in a container, then the iterators [first, last) should not be equal. "first" is an iterator located one element to the left of iterator "last".

If first == last, then the span of possible iterators is:

[first, first) or [last, last)

The possible span cannot both include and exclude any iterator. That sounds crazy.

Good luck.